MODULE SUPPLEMENT: ENDOCRINE SYSTEM
Why Does Growth Hormone Decrease?

As you can see from the table below, many factors are involved in growth hormone release or inhibition.

The various factors involved in the stimulation and inhibition of growth hormone work mainly through their effect on GHRF and GHIF (somatostatin). There is evidence that there is a decrease in GHRF and an increase in GHIF (Ruiz-Torres & del Melo Kirzner, 2002). Much of these data are from animal studies, although human studies support the findings with some indication that the exact mechanisms may differ between men and women (Muller, Rigamonti, et al., 2002; Orrego, Russell-Aulet, et al., 2001; Russell-Aulet, Dimaraki, et al., 2001). If these neuro-hormones do change, it follows that the secretion of growth hormone would be significantly affected.

Of note is that the administration of GHRF can normalize growth hormone section and levels of somomedin C, suggesting that pituitary responsiveness to GHRF is probably not decreased. Further, data suggest that, at least in men, peripheral responsiveness to Insulin-like-Growth Factor is maintained (Lissett & Shalet, 2003). Thus, there is currently no evidence that feedback inhibition, plasma clearance, or the peripheral action of growth hormone changes significantly with age.